1st : your tactics is too defensive to score...
2nd : Mertesacker isn't high wage like tulaugu... He is a fantastic defender

But unfortunately considering you are in your 3rd season now, I reccomend you start getting replacement in for him by the end of your 4th season... He is still one of the better defenders for me in my 5th season in arsenal. Maybe time to bring in one of the young defender you are grooming into your team?
3rd : When grooming young player... Maybe you didn't notice but keeping them at the club and tutoring them while also playing them some matches like cup matches or easy to win league matches will help them develop faster than loaning them out. Thats how I get my team full of youngsters I develop by the 5th season and I am running on a really low wage for a CL winning team.
4th : I can assure you that your team has world class defense... Don't know what that guy has against kos, vermaelen and mertesacker... They are world class defenders in the game in fact kos is a very versatile player in the defense role... If you need a full back/wing back, he can actually be played as a full back/wing back and he can be place them and I assure you he will deliver a amazing performance.

For the record, mertesacker, vermaelen and kos(LB) was on the UEFA dream team on my save for the 3 CL I won... So I don't get why they are not world class? They been keeping my defend totally solid. I got a feeling you use them wrongly tulaugu... Zouma can't even do better than how mertesacker did when mertesacker was at his peak on my save...
5th : Walcott isn't that good like he said... But he is a good player to keep for a backup tbh... But sell him if you want to.
6th : If you want some kick in the game... TBH dun follow his advice... If not you might as well buy all the best regens you find by this point... Better than buying those players he reccomend...(Except zouma and veratti)
7th : Yes you might want to try a new tactics, you do have many good attacking mids.
8th : If you need a player on the LWB I would reccomend david alaba
I think you got a nice squad there just that the tactics isn't working... Sorry I am not really a expert when it comes to working with formation with 3 CBs... I usually work with 2 CBs or even just 1 CBs... I am more of a fast attacking philosophy person.